Spotted Wing Drosophila Emergency Meeting Results
Ξ August 30th, 2009 | → 0 Comments | ∇ A Day at a Time, Technology, Wine News |
Planned only a week and a half before, University of California Cooperative Extension (UCCE) Farm Advisor Mark Bolda’s August 26th emergency meeting in Watsonville on the Spotted Wing Drosophila (SWD) was thorough and informative. His up-to-the-minute research presentation, and that of his charming colleague, Martin Hauser, Associate Insect Biosystematist with the Plant Pest Diagnostics Lab, California Department of Food & Agriculture (CDFA), Sacramento, provided the assembled growers, foremen, and associated persons, representing an estimated 75% of the caneberry acreage in Santa Cruz county, with insight into the history of the pest, the fruits under threat, the critical importance of early detection and identification, bait and capture techniques, the basics of field sanitation, and pesticide options for both the conventional and organic producer. But much work remains.
Mr. Bolda began with the acknowledgment of the group effort required for his summation. “I am standing up here by myself but in no way or form did I do all this work on my own. I had a lot of support from people in the industry.”
What follows is a distillation of his talk, followed by that of Martin Hauser’s. My focus, of course, is on the wine industry, of the pest’s threat to vineyards. Hence, my notes taken from Messrs. Bolda and Hauser will faithfully report what might be the basics that a vineyard manager ought to know in advance of a potential crisis. For on-going research and breaking news, Mr. Bolda’s farm blog, Strawberries and Caneberries is a ‘must read’. And it is with his work that I begin.
History of the SWD in California
In 2007, in a vineyard in Paso Robles a suspect grape was picked up, but as no specimens were kept, it remains a rumor that SWD was the agent.
Late summer of 2008, in over just a few weeks, multiple reports came in of the presence of vinegar larvae in unharvested cane fruit. Something was different. Research established it was a different species of vinegar fly than hitherto known. Samples were sent to CDFA. That it was in the family of Drosophilidae was confirmed, but the matter was not further pursued. Fruit producers, however, knew it was a damaging pest. Independent studies, without CDFA’s assistance, were initiated by concerned growers and the UCCE.
Spring 2009. Colossal infestations of the new vinegar fly in cherries were reported. People sat up and took notice. It was then provisionally named the Cherry Vinegar Fly. The cherry industry was not pleased so the name was soon changed to the Spotted Wing Drosophila. It has now been picked up in plums, strawberries, blueberries, blackberries, raspberries, and olallieberries, but not yet in wine grapes. Additional infestations have been reported in Oregon and Washington.
The male is identified by the presence of black spots on the wings. For most people the female looks like a regular vinegar fly. But should spotted wing males be present in a field the presence of females may be confidently extrapolated. [Photo by Dr. M. Hauser]
SWD is native to South East Asia: India, Bangladesh, South East China. It has spread to Japan, Korea, it is in Hawaii, Florida, California, Oregon, Washington, British Columbia. Neither the CDFA or the USDA has given any indication that this will become a quarantine pest. The entirety of California was infested at the same time, therefore it is not possible to create a quarantine around it. Because eradication is not possible the effort will be to bring down SWD to manageable levels.
Life Cycle
Fruits attacked by SWD reveal only a soft spot on its surface, slightly indented. No hole is casually visible. [See Mark Bolda's blog for pics.] When the fruit is cut open at the sunken spot a larvae may be observed. The female lays one to two eggs per fruit, with an overall capacity to lay 300 eggs. More than two larvae indicates multiple females laying eggs on the same fruit. The damage of the fly is not the feeding by the adult on fruit but from larval development.
Normally fruit flies most people are familiar with lay eggs on the surface of things. The ovipositor is benign. It does not insert eggs. The SWD, by contrast, possesses an ovipositor designed to saw through the surface of fruit. Not a needle-point but a saw-like structure. An egg’s deposit depth is believed to be approx. 1 mm or 2 mm.
The colder it is the longer it takes for the SWD to develop. At 54 degrees it takes 50 days to develop from egg to adult. At 64 degrees it takes 19 days to mature. At 77 degrees 8.5 days; 82 degrees, 7 days, one generation a week. Beyond 85-86 degrees the males become sterile. Reproduction ceases at that point.
Monitoring and Management
Early detection is critical. As is the use of preventative sprays and enhanced sanitation protocols. Cane berry growers are especially hard hit owing to the practice of closed canopies and tunnels. Once SWD is well established it becomes very difficult to control them. Sanitation methods currently enjoyed are insufficient.
SWD is a very mobile pest. It is felt trapping is the best method for early detection. Early expert advice (Dec. 2008) suggested using banana slices placed at the base of the canes. Extremely limited success was achieved. Next was banana slices and apple juice in a mason jar hung on a stake in a raspberry field. It worked far better trapping SWD but it required a lot of service by the larger grower. More efficient trapping methods were sought.
The next trap experiment used was GF120, a fruit fly bait, this time hung at a lower level in the canes because the SWD does not like the sun, it prefers shade. The bait worked moderately well but its attractiveness to the fly declined over time.
What do the adult flies like to eat? They like old stuff, fermenting materials. Again, only the larvae are destructive. Of other attractants, a strawberry purée or GF 120 were still working; methyl eugenol, currently advocated by the CDFA as a bait, did not prove effective in field experiments. In fact, it never picked up a single fly.
(It is important to add that only the male SWD was used as an indicator of their presence in the field. The females are simply too similar to vinegar flies when examined in the field to be of much diagnostic help.)
Other fruit purées were explored. Yeast and sugar mixed with water proved the most effective bait (one package of Baker’s yeast, four teaspoons of sugar and 12 ounces of water). Mr. Bolda then offered traps (see pic) to folks. He had assembled them for use in SWD detection with the recommended bait solution mentioned above to be added later.
Note that the sugar is added not for the SWD but for the yeast. The idea is to produce a rapidly fermenting liquid attractive to SWD. The monitoring traps are then placed throughout the field. Attention should also be paid to the direction the SWD appear to be coming from.
Control
GF 120 (not a spray, but OMRI certified). Sprays: Spinosads (permitted for organic use, [but broad spectrum]), Mustang, PyGanic (permitted for organic use), and Malathion were all discussed. As was the importance of preserving predators and parasitoids.
Mustang proved very effective, as did Malathion, even after five days. Spinosad (Trust), gave good results but lost efficacy over that same five day period. PyGanic was of minor efficacy.
A single application is not enough in a heavily infested field. Three applications, one every five days, was recommended to break the cycle. But early detection may reduce the need for repeated applications.
Rotating the chemicals was stressed. Without rotation resistance will develop. And these are not the only sprays available. The ones tested were ready at hand. The idea was to provide immediate research results into the efficacy of some commonly available pesticides. Other chemical alternative may well, indeed, probably exist.
March, 2010 update on Control of SWD An email from Mark Bolda reads, “We did find just before the holidays that spinetoram is pretty useful on SWD, giving some 3 weeks of control at least.”
Sanitation
Dropping fruit to rot in the field is strongly discouraged. As is leaving incompletely harvested fruit. If not disced, the discarded fruit must be removed from the field and be physically destroyed.
Mr. Bolda stressed that the berry industry is only as strong as its weakest link. All growers must do their part to implement the research findings.
—–
Here now is a partial summation of the talk given by CDFA’s insect specialist, the very engaging and downright hilarious, Martin Hauser. It was his primary research that led to the identification of this new California pest, what is now known as the Spotted Wing Drosophila. [Martin Hauser's PDF to come.]
He acknowledged Mr. Bolda’s thorough presentation. His talk was to be a kind of positive reinforcement.
CDFA History of SWD
Last September UCCE sent flies to the CDFA. They were identified to genus level as a Drosophila and was considered completely harmless. The larvae always develop in rotten fruit, some in fungi, occasionally elsewhere. They normally eat the fungus which develops in rotten fruit. Again, the first estimation of the flies sent were considered completely harmless. CDFA was wrong.
Spring brought more and more calls from growers, especially of cherries. After a close look at the larvae specimens sent in it was again determined by the CDFA to be a harmless Drosophilid. Mr. Hauser said that in his defense it was rather like hearing from ranchers that there were rabbits attacking and eating their cows. Rabbits don’t eat cows! (laughter) They eat carrots. After a while one no longer believes these people, these ranchers.
A summary was given of other dangerous agricultural flies. But the fruit and vinegar flies were, up until now, thought to be drawn only to rotten fruit. Of no real economic importance. Though a single fly which falls into your glass of red wine can ruin it.
The larval samples initially received from cherry growers were too difficult to identify. Taxonomic identification could not proceed beyond the Family level. Indeed, there are 3,000 different species of Drosophilids in the world. And nobody over 100’s of years of agriculture in California, nobody ever reported or described the SWD here. It must be coming from outside. With increased trade and shipping the fly could be from anywhere in the world. So began a search through the scientific literature on the 3,000 species.
Identification
Only 125 Drosophilid species live in North America. There are 600 species in Hawaii. The islands’ isolation, abundant fruit, and absence of competitors allowed considerable speciation. But they are all harmless and restricted to Hawaii. The SWD could not be one of those well-described species. So, Dr. Hauser’s job is to identify insects, to give them a name, the scientific name. And this step is crucial. Everything is connected to the name. The scientific name is the key to the literature. One can then learn of a pest’s predators, parasites, biology, among many other things.
Deep in the literature was found the first description of California’s newest pest, suzukii (the Genus [Leucophenga] was not proper). In 1931 Matsumura Suzuki described the species as new to science in Japan. Just a few years later the species was then described as a pest in Japan. Dr. Hauser’s theory is that the species was not native to Japan. It was introduced from elsewhere, South East Asia, perhaps.
When a new species of insect is first introduced to a new area you have a massive, invasive explosion. It just eats. It has no natural enemies. There follows a few years of explosive populations. Then pathogens and parasites move in. The invasive insect population then begins its decline. In the Japan of today the SWD still causes trouble but not the catastrophic trouble California et. al. have here right now.
Description
The male fly has spots on its wings. There is no other Drosophila in North America with spots on its wings. Other flies have spots, there can be some confusion, but this characteristic makes it easy for even the relatively inexperienced person to identify them. No microscope needed. The females are harder to identify. They are relatively big and heavy Drosophilids. The large ovipositor is key. With it the female cuts through a fruit’s skin “like butter”.
Life Cycle
The eggs are very small. They cannot be seen in the field. There are three instars of increasing size. The third, the largest, is the one most often seen because the damage already done to the fruit has now become easy to see in the field. Then there is a pupa, the last transitional form to the fly.
The average time for one generation is about 12 days. [Mark Bolda's collaborative field research must take precedence here. See his comments above.] From adult to another adult, 12 days. Varies with season and temperature, as Mr. Bolda’s work shows. The female lays 350 to 400 eggs. The egg stage is between 12 and 72 hours, a very short time. Larval stages are between 3 to 14 days. The pupa, 3 to 15 days, depending on the temperature.
Distribution
The fly is originally from Asia, very likely China. But also Japan, Korea and Thailand. When calling colleagues around the world Dr. Hauser heard from an entomologist in Switzerland that he had just found it in Spain last year! It was also found in the early 2000’s in Hawaii, in fruit fly traps. It was not attracted to the fruit fly attractant but to the traps already full of fruit flies. It seems the rotting fruit inside the fruit flies attracted the Drosophilid. But there are no reports of agricultural damage there. The same for Spain.
The presence of SWD was detected in San Diego, all over the LA Basin, they are everywhere along the Coast and the Central Coast, the Central Valley up to Sacramento, the Bay Area. A lone maggot was trapped in Humboldt. Suzukii was recently detected in Florida. Dr. Hauser predicts SWD is all throughout the mid-west owing to shipping and distribution patterns. A fruit may be deemed sub-standard and tossed into the dumpster, for example, thereby allowing SWD to reproduce. The fly will eventually be found in every state with significant fruit production and that is not too cold.
As a side note: CDFA used Methyl Eugenol which attracts male fruit flies through mimicking the scent of the female. The male climbs into the trap and drowns. A good lesson, Dr. Hauser suggests! But the reason SWD is attracted to M. Eugenol is because of the presence of already fermenting fruit flies. This attracts the Drosophilids into the fruit fly traps. Therefore, the primary attractant that is used for fruit flies does not work, absent rotting flies, with SWD. It is agreed the use, the addition of yeast is ideal.
Natural History of SWD
A Japanese paper from 1939 was found describing the biology of the SWD. Eyebrows were raised when it was read that the fly infests cherries and grapes severely. Also apple, peach, plum and persimmon. Cherry infestation reached as high as 75% according to the ‘39 paper. SWD can have 13 generations in a year. They are active all year round in Japan. (And here. There is no snow or severe weather. Further up north they might try to hibernate. They probably could survive a ‘normal’ winter.) They are hardy flies. But the 1939 report also held out promise: it seems the larvae are parasitized by a specific wasp. The CDFA is looking hard to find natural enemies in other countries and to import them here for enhanced natural control.
What will it hit next? Cherries have been hit hard. The Spotted Wing Drosophila was initially called the Cherry Vinegar fly. But neither the Cherry industry nor the Vinegar industry wanted a fly named after them! Perhaps call it the Japanese Fly? No. The Japanese might not like this. So CDFA came up with the Spotted Wing Drosophila. It is also found in raspberries, strawberries, and recently, plums, Asian, Satsuma and Plumcots, blackberries, boysenberries, and surprisingly, nectarines. This is what has been found in California. But it is not thought they will go into apples or oranges.
And of grapes? This is always floating around. It is kind of a political issue. There have been reports of tomatoes, apple and apricots. Sometimes you can’t really trust these literature reports. People may have just found a maggot. The reports must be taken with a grain of salt pending DNA analysis. This is just the potential of the fly. There is still no proof they go into grapes.
10/13 This has changed. Please see this update.
UC Davis may have done some experiments where grapes were offered to SWD. They more or less forced them into the grapes. They gave SWD no alternative. Eventually the females laid their eggs where the stem enters the grape and grapes were infected. But the experiment was unnatural. It is still unclear whether SWD is a danger for grapes. There is no confirmed damage to wine grapes in Nature, so to say. Why might that be? It is felt grapes have a stronger skin the SWD cannot really penetrate. But there are many different varieties of grapes. Some may prove more susceptible than others. Dr. Hauser felt it improper to exclude healthy grapes at this point. SWD, however, does go into already damaged or rotting grapes. It is not clear whether SWD caused the damage or arrived after the grapes had already been corrupted in some way. More work needs to be done.
The USDA is currently taking no regulatory action because the SWD is already everywhere. And in a few years it will be everywhere in the world. Everybody will have it. There will be, therefore, no export restrictions on cherries to Japan, for example, because they already have the SWD.
END
Special thanks to UCCE’s Mark Bolda for extending me an invitation to the Watsonville meeting.
Admin
Here is a supplemental piece of some interest. More to come.
